This invention relates to a transmitted-reference, delay hopped (TR/DH) ultra-wideband radio communications system and, more particularly, to a method of initial synchronization, or acquisition, of ultra-wideband (UWB) communications having other modulation formats by means of a transmitted-reference preamble.
Ultra-wideband is a form of radio communications that operates by transmitting very short pulses, or radio frequency (RF) bursts. At the present time, the most well-known method of UWB communication uses the pulse position modulation (PPM) scheme disclosed by M. Z. Win and R. A. Sholtz in “Impulse radio: how it works”, IEEE Comm. Letters, vol. 2, pp. 36–38, February 1988, and by L. W. Fullerton, “Spread spectrum radio transmission system”, in U.S. Pat. No. 4,641,317. This scheme is used, for example, in optical communications, and it is described in many standard textbooks, such as J. G. Proakis and M. Salehi, Communications Systems Engineering, Prentice Hall, 1994, pp. 439–442. The method works by transmitting and receiving pulses in one of a number of time slots in successive time frames, which requires close time synchronization between the transmitter and the receiver.
The pulse position modulation (PPM) scheme is sometimes called a “time modulated” UWB method, because the relative time of arrival of the pulses is important information that must be known by the receiver in order to demodulate the transmission. Other methods of time modulated ultra-wideband modulation are also possible. For example, the phase or polarity of a received pulse could be used to represent the transmitted information. In such a scheme, the time between transmitted pulses may be modified in a pattern known to both the transmitter and receiver. The scheme just described is known as time hopping, and it is described in M. K. Simon, J. K. Omura, R. A. Sholtz and B. K. Levitt, Spread Spectrum Communications Handbook, Revised Edition, McGraw-Hill, 1994. The time hopping scheme provides the time modulated UWB transmission with a certain level of multiple access capacity; that is, it allows for multiple transmissions to be simultaneously demodulated.
The time hopping is also the multiple access method used with PPM ultra-wideband. See R. A. Sholtz, “Multiple access with time-hopping impulse modulation”, Proc. IEEE MILCOM '93, Boston, October 1993. When time-hopping is used with PPM, we refer to the overall communications method as PPM/TH ultra-wideband. This scheme has been promoted as giving a very large multiple access capacity to the UWB channel, even at high data rates.
One problem with any time modulated UWB transmission scheme is the difficulty of establishing time synchronization. This is especially true when the transmissions consist of bursts of transmitted symbols, in which case the receiver has no a priori information whatsoever about the transmitters timing, and, in addition, it must detect the presence of the transmission prior to attempting to demodulate it. (On the other hand, if the transmissions are imbedded within some kind of data packet framework, the receiver might know approximately when a transmission is to start.)
In the absence of multiple access interference (MAI), the clock governing the pulse slots can easily be recovered from the received signal, while the frame clock must be recovered from a transmitted acquisition sequence. This problem has received some attention in the literature. See R. Gagliardi, J. Robbins and H. Taylor, “Acquisition sequences in PPM communications”, IEEE Trans. Information Theory, vol. IT-33, pp. 738–744, September 1987. However, multiple access interference worsens the problem significantly, since the pulse-slot clocks of the separate interfering transmitters will not be synchronized with that of the desired transmitter. This situation requires the acquisition of both the slot clock and the frame timing through the use of an acquisition sequence. This requirement means that the initial synchronization is very computationally demanding, and this computational burden can result in either a very long acquisition time, a very expensive receiver, or possibly both.
Recently, a new alternative UWB communications scheme, called transmitted-reference, delay-hopped (TR/DH) ultra-wideband, as been invented, as described in copending patent application Ser. No. 09/753,443. The term “transmitted reference” refers to the transmission and reception of multiple pulses in such a manner that synchronization with the individual pulses is unnecessary. Transmitted reference UWB transmits pulses in pairs, and thereby induces a correlation at the receiver that can be measured by standard means. The term “delay-hopped” refers to a code-division multiple access scheme which uses transmitted-reference UWB.
In addition to the standard ultra-wideband (or “impulse radio”) version of TR/DH, the inventors have invented and experimented with a version of TR/DH that uses wideband noise as a carrier, rather than impulse trains. This version of the invention induces correlation at the receiver by transmitting the sum of two versions of a wideband continuous noise, separated by a lag known to the receiver. This invention has advantages in that the noise carrier may be easier to generate than the impulse train carrier, and it is described in copending patent application Ser. No. 09/974,032.
Although the TR/DH scheme does not have the synchronization problem of time modulated schemes such as PPM/TH, its signal-to-noise ratio (SNR) is worse and it is more susceptible to multiple access interference. On the basis of predictions of performance based on mathematical modeling and computer simulation, these effects appear to limit the multiple access capacity of TR/DH to the degree that it will not be suitable for some applications. It would be advantageous to combine the low-complexity synchronization properties of TR/DH with the multiple access capacity of a time modulated UWB method, such as PPM/TH.